Hi,
in the framework of a project of monitoring environmental conditions within a cave
I need to connect two or more arduinos using just a long piece of telephonic cable.
So I just have 1 wire for ground and 1 wire for communication.
The length of the cable is from some hundreds of meters up to some km.
The baud rate can be low, I have to monitor environmental parameters which changes slowly in time.
I think to setup for this task an half-duplex line, with one ARDUINO kept outside the cave as master and the other ARDUINO
inside the cave working as slaves
I looked for RS485 protocol but it need three wires : RX,TX and GROUND, so it can be used.
Is there anybody able to suggest to me a solution?
Consider that:
the environment of caves is particularly harsh. I need rugged electronics and cases and all has to be light,
strong and watherproof.
lucklily linghtnights inside caves are not frequent, if the master "outside" cave is located a couple of meters undergroud it is effectivelly shielded.
wireless communication is in general not feasable, caves have deep (30 - 100 mt) pits, interconnected by narrow (80cm) passages, no way to have a safe signal coming out.
the dream would be to have slave arduinos powered from outside, through the same bipolar cable used for communication,
but may be this can be a second step improvement
http://www.maximintegrated.com/app-notes/index.mvp/id/3884
"Proper wiring[6]: RS-485 specifies differential transmission, which requires two signal wires in addition to a ground wire (commonly a 24AWG twisted pair) to transmit the signal. The two signal wires carry signals opposite in polarity, and greatly reduce the problems of radiated EMI and EMI pickup. The common characteristic impedance of this wire is 120?, which is also the resistance used to terminate each end of the cable—in the interest of reducing reflections and other transmission-line effects. Figures 7 and 8 illustrate properly wired systems. "
Since you only have Gnd and one wire, what you need is for the master to transmit while the slave recieves, then turn off its driver and listens, while the slave finishes listening and enables its transmitter to send out.
One way to do that is to use a simple open collector buffer. Each end drives the input to the buffer and also monitors its output when it is not transmitting. A pullup resistor pulls the line high, and all either end can do is pull the line when they are transmitting.
A simple SN7406 can do this
Either end can bring the signal low while transmitting, the resistor brings it high.
MicheleMaris:
I looked for RS485 protocol but it need three wires : RX,TX and GROUND, so it can be used.
Over that distance, I don't believe you have any real choice to use less than a three wire RS485 solution. You need the differential twisted pair (two wires) for signal noise rejection, and the the ground wire (third wire) to provide a common reference ground at each end.
Over those distances, the voltage of "ground" for each device can vary by several volts. Indeed, if ground voltage varies by enough to exceed the absolute signal voltage limits at the interface, optical isolation will be required as well so as to not damage the interface.
I really think you will find two wires is a lost cause. Three wires and RS485 is the engineering solution that has already been worked out for you to solve this problem.
MicheleMaris:
Consider that:
the environment of caves is particularly harsh. I need rugged electronics and cases and all has to be light,
strong and watherproof.
You might also consider a more rugged board than an Arduino. As prototyping boards, they are not designed for extreme conditions in terms of temperature and/or humidity. However, there are a range of rugged, compatible clone devices sold by third party vendors, specifically designed for deployment in harsher/industrial environments. I would recommend doing some further research into these options to determine what board(s) might be better suited to the particular harsh conditions you describe.
Not sure if this will help, (not sure if you are using Ethernet to communicate or want to) but I use Ethernet extenders that use only one pair to send data long distances. They seem to cost over $200 for a pair but they are amazing. By using a higher voltage than regular Ethernet, these things can combine transmit and receive signals and send them over long distances. The link below is not to a device I am explicitly familiar with. It claims to send up to a distance of 1 KM - that's a normal claim. If interested, I can find my model # of one I know works well.
Basically, the device takes a standard 4 pr. Ethernet cable from one of your Ethernet shields, then can send over a 1 pr. phone line to the other device, plug in a Ethernet cable on that end to your Ethernet Shield.